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[Python-Dev] Second post: PEP 557, Data Classes
Eric V. Smitheric at trueblade.com
Tue Nov 28 13:31:39 EST 2017
On 11/28/17 7:02 AM, Nick Coghlan wrote:> On 28 November 2017 at 17:41, Eric V. Smith <eric at trueblade.com> wrote:>> One thing this doesn't let you do is compare instances of two different>> subclasses of a base type:>>>> @dataclass>> class B:>> i: int>>>> @dataclass>> class C1(B): pass>>>> @dataclass>> class C2(B): pass>>>> You can't compare C1(0) and C2(0), because neither one is an instance of the>> other's type. The test to get this case to work would be expensive: find the>> common ancestor, and then make sure no fields have been added since then.>> And I haven't thought through multiple inheritance.>>>> I suggest we don't try to support this case.>> That gets you onto problematic ground as far as transitivity is> concerned, since you'd end up with the following:Excellent point, thanks for raising it.> >>> b = B(0); c1 = C1(0); c2 = C2(0)> >>> c1 == b> True> >>> b == c2> True> >>> c1 == c2> False>> However, I think you can fix this by injecting the first base in the> MRO that defines a data field as a "__field_layout__" class attribute,> and then have the comparison methods check for "other.__field_layout__> is self.__field_layout__", rather than checking the runtime class> directly.>> So in the above example, you would have:>> >>> B.__field_layout__ is B> True> >>> C1.__field_layout__ is B> True> >>> C2.__field_layout__ is B> True>> It would then be up to the dataclass decorator to set> `__field_layout__` correctly, using the follow rules:>> 1. Use the just-defined class if the class defines any fields> 2. Use the just-defined class if it inherits from multiple base> classes that define fields and don't already share an MRO> 3. Use a base class if that's either the only base class that defines> fields, or if all other base classes that define fields are already in> the MRO of that base classThat seems like a lot of complication for a feature that will be rarely used. I'll give it some thought, especially the MI logic.I think what you're laying out is an optimization for "do the classes have identical fields, inherited through a common base class or classes", right?Eric.
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